There seems to be a dearth of VT420-specific emulation packages (a la Kea! on Windows) for the Mac full-stop.
I have to dial into an Alpha VMS box for work and do it one of 2 ways, depending on what I need to do. I either dial into our network RAS server and then telnet in from the terminal (which has some limited VT100 support - even more limited in 10.1 than it was in 10, incidentally), or I use ZTerm, to dial directly into the VMS box.
ZTerm has the advantage of letting you write macros, so you don't have to remember the ridiculous Esc[12~ commands to perform the various VT420 keyboard functions, and it'll emulate a VT keyboard (like the Terminal used to - grrr). Unfortuately, it will only support one screen size, but then so does the Terminal now (grrr again).
Either of these options should provide you with enough functionality, but if you find a dedicated VT420 emaulator, do let me know!!